Laser sintering of Au nanoparticles on a Cu substrate was studied toward an alternative to gold plating. The laser sintering by laser direct writing method required much higher laser power density in comparison with that on a glass substrate. The laser-induced heat accumulation method is proposed as a new laser sintering method which can be applied to a metal substrate with a high thermal diffusion coefficient such as a Cu substrate. In the sintering process, the dissipation through the Cu with a high thermal diffusion coefficient was reduced by using a thermally isolated Cu substrate, where the heat accumulation in the Cu micropattern was caused sintering of whole area by laser pulse irradiation at a small spot without laser beam scanning. The SIMS profile showed that the diffusion of the Cu atom through the gold layer was reduced remarkably by the laser sintering compared to the conventional heat treatment. The reduction of Cu diffusion into gold layer is due to the extremely short sintering time of the laser-induced heat accumulation method.